Use of a laser light source as a detector via a dithering modulation

ABSTRACT

Laser light is focused in an accurate manner on selected layers in a multilayer optical disk recording system by means of an analysis of output electrical signals from a dithered source which results in the production of an electrical signal which is proportional to an amount by which the parts of the system are to be relatively moved in order to achieve the desired focus on one of the layers.

BACKGROUND OF THE INVENTION

The present invention is generally directed to a system for focusinglaser light on a selected layer in a multilayer optical disk informationstorage system. More particularly, the present invention is directed toa system for providing an electrical signal which is proportional to anamount by which a laser light source is to be moved relative to arotating optical disk in order to achieve focus. Even more particularly,the present invention is directed to a system in which a lens or lightsource is electromechanically dithered to produce an electrical signalwhich is distinctive in terms of its ability to provide an indication ofthe optical layer which is being read or written to. Most importantlyand most relevantly to the present invention, it is directed to theutilization of a semiconductor laser as both a light source and adetector.

Use of optical disks for information storage has become quite popular.These disks are capable of storing digital data and are generallyreferred to as CD ROMS. These devices have been shown to be capable ofstoring many hundreds of megabytes of information.

Even more recently, it has been shown that it is possible to write andread information to multiple layers using optical disk storagetechnology. These new optical disks can store distinct information oneach additional layer. This multilayer approach to optical storagesignificantly increases the storage capacity of each disk. However, newhigh-density drives are required to utilize these disks.

These new drives contain a laser and a lens mounted on a servomotor-controlled mechanism. By moving the lens toward and away from theoptical disk, laser light is focused on different layers in the disk.However, the more layers that are employed in the disk for data storage,the more difficult it is to provide proper laser focusing. Moreparticularly, as a result of the focusing difficulty, these diskstypically require "buffers" of 100 microns or more in thickness betweeneach of the data layers.

Accordingly, there is a need to be able to accurately focus laser lightonto the optical disk and concomitantly to be able to read reflectedlight from that particular layer. It is, therefore, seen that it wouldbe highly desirable to be able to employ an alignment technique usingthe servo motor and feedback loop which are already present in suchdrives. Furthermore, it is generally desirable to be able to improvetracking even for single layer optical disks in order to make them moretolerant to shock and vibration for use in portable or mobileapplications. It is also very desirable to be able to provide a solutionto these problems via a simple modification of disk drives which arealready in use so as to improve their tracking accuracy. These needs areparticularly well met by a system in which the source of laser lightalso acts as the detector for reflected light.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, an opticalinformation storage system comprises a rotatable optical disk whichcontains stored information. A source of laser light is provided so asto produce modulated reflections from the rotatable disk. A lens systemmeans or mechanism is provided for focusing the laser light in thevicinity of the optical disk. A dithering means or mechanism is providedwhich causes oscillation of the source of laser light and/or the lenstoward and away from the rotatable disk. This causes a modulation in thedetected signal which can be used to significant advantage. Furthermore,the system employs the laser light source as a self-contained source anddetector which receives light after it has been reflected from the disk;the light source/detector converts the light to an electrical signal.Furthermore, and for particular relevance with respect to the presentinvention, there is a means or mechanism provided for analyzing theelectrical signal from the light detector and for producing therefrom anelectrical signal which is proportional to an amount by which the lightsource should be moved relative to the disk in order to achieve desiredfocus. In an alternate embodiment of the present invention, instead ofmoving the light source, it is also possible to change the focus of thelens to achieve the same objective. In another embodiment of the presentinvention, the means for analyzing the electrical signal from the lightdetector may be employed simply to indicate which plane the laser lightis focused on in the multilayer CD-ROM disk.

The inventors herein have discerned that the signal detection processmay be based upon a heretofore undescribed physical phenomenon thatoccurs in certain solid state laser devices. In particular, the presentapplicants have discerned that the current in a laser diode varies inintensity when its own modulated light is reflected back into it. Inparticular, the present applicants have discerned that it is possible toemploy the same laser diode as both the source and the detector for aCD-ROM drive circuit. In particular, the present applicants havediscerned that this phenomenon permits the circuit to be constructedwithout a separate photo detector. Accordingly, this results in lighterweight, easier to move, and easier to focus laser light/detectormechanisms. Furthermore, the result is that increasing optical storagedensities are possible.

It is further noted that a significant aspect of the present inventionis the utilization of specific output waveforms which may be employed todiscern the positioning of the laser source. In-particular, through thesimple expediency of a thresholding circuit coupled with two counters,it is possible, through appropriate computer logic, to provide anelectrical signal which indicates the focus plane. Furthermore, as anatural consequence of being able to know where the focal plane islocated, it is concomitantly easier to, therefore, control where onewants it to be.

Accordingly, it is an object of the present invention to provide amechanism for reading and writing information stored on multilayeredoptical disks.

It is, furthermore, an object of the present invention to provide adigital electrical signal and circuit which more accurately andprecisely aligns laser reading and writing mechanisms in optical diskmemory systems.

It is yet another object of the present invention to more easily controlfocus and alignment in CD-ROM systems.

It is also an object of the present invention to increase the storagecapacity of optical information disks.

It is still another object of the present invention to enhance theutilization of multilayer optical disk and the performance of multilayeroptical disk systems.

It is also an object of the present invention to make the laser writingand reading mechanisms lighter in weight.

It is also an object of the present invention to increase the storage,speed, and capacity of optical disk drives.

It is a still further object of the invention to employ Z-directionfocusing principles to also provide accurate track following in the Xand Y directions.

It is yet another object of the present invention to provide a simpledigital circuit for determining focal positioning.

Lastly, but not limited hereto, it is an object of the present system toprovide a mechanism which is capable of acting both as a laser lightsource and as a reflected laser light detection mechanism.

DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with the further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 is a partially schematic block diagram illustrating the system ofthe present invention;

FIG. 2 is a functional block diagram illustrating a signal processingmechanism for producing a signal which is proportional to-the distanceneeded to provide proper focus;

FIG. 3 is an illustration of the various signal waveforms produced inaccordance with the present invention as a function of the DC positionof the focal plane with respect to the waist plane of the lens;

FIGS. 4B-4F are illustrations of the signal waveforms illustrated inFIG. 3 subsequent to their processing through a threshold detectioncircuit; and

FIG. 5 is an illustration of one embodiment of a digital circuit whichmay be employed to determine the position of the focal plane;

FIG. 6 is an illustration of an alternate embodiment of a digitalcircuit which may be employed to determine the position of the focalplane; and

FIGS. 7A-7D are illustrations of the various signals produced byreflected and detected (laser) light when dithering is employed to trackalong a boundary in the X-Y plane between areas of differingreflectivity.

DETAILED DESCRIPTION OF THE INVENTION

Instead of attempting to maintain focus by stepping the lens from oneposition to another, rather, in accordance with the present invention,the lens is moved back and forth or is "dithered" about a centralposition. The dither frequency may be as low as a few hundred hertz soas not to affect other aspects of the disk drive operation. The ditheredlight is reflected from the optical disk and sampled with aphotodetector. Alternatively, the laser or LED itself may be used as aphotodetector in accordance with other aspects of the present inventiondiscussed in greater detail below.

When the optical disk surface to be read is in focus, the reflectedlight signal frequency is doubled with respect to the dither frequency.Doubling is more particularly illustrated in FIGS. 3 and 4 discussed indetail below. This frequency doubling can be detected with athresholding circuit which converts the frequency doubling into adigital signal. When the lens moves to focus on a different layer of theoptical disk, the reflected light signal is only partially frequencydoubled. It is possible to determine accurately when the light isfocused on several different planes in the disk using this method.Spacing between successive planes on the disk is equal to one fourth ofdither signal. For example, if a 2.5 kHz dithering is used, it ispossible to distinguish between focusing on two planes which are 100microns apart. A slightly higher dithering frequency of 5 kHz enablesone to resolve two planes which are only 50 microns apart. In this way,one can focus light on more planes which are spaced more closelytogether. The invention is limited only by the practical ditherfrequency of the lens. Nominal position of the lens (center point of thedithering) is actively controlled by a feedback loop from a detectorcircuit. In this way, the desired plane of the disk is always held infocus. By maintaining more accurate focus on the disk, the presentapproach allows the use of more layers on a single disk and can increasethe storage density by a factor of 3 or 4 or more. As an alternative,the present invention may be used to detect with high accuracy whetheror not a single layer disk is in focus.

Accordingly, the method of the present invention is also applicable tothe manufacture and utilization of single layer disk drives which are,therefore, more robust and resistant to shock and vibration such asmight be encountered in mobile and/or portable computer environments.Furthermore, the implementation of the present invention is exceedinglypractical in that existing optical disk drives already contain thenecessary servo motor and lens positioning system which are availablefor implementing the present invention. It is further noted that thepresent invention is applicable to the reading and writing of opticaldisk drives which store computer data, audio information, and/or videoinformation. Furthermore, it is noted that the dithering provided by thepresent invention may also be produced by a mechanical oscillatorydistortion of the lens itself rather than employing a mechanicalapparatus to simply move the lens backwards and forward. However, thisis not the preferred embodiment of the present invention.

For a more complete understanding of the present invention, attention isdirected to FIG. 1 wherein there is shown, in schematic diagram form, anarrangement which may be employed to illustrate the operation of thepresent invention. In particular, in FIG. 1, there is shown diodeinjection laser 10 which directs laser light through preferred crossedfiber 80 which corrects for unequal divergence of the laser light intolens 100. Crossed fiber 80 serves to circularize the laser light so thatthe divergence in both directions is equal. The physical positioning oflens 100 is controlled by means of voice coil 20 which is provided withan appropriate dithering signal from function generator 60. This signalalso includes an appropriate DC offset control level. Light from lens100 is directed onto optical disk or CD target 30 from which it isreflected back through lens 100 and diode injection laser 10 tophotodetector 40. The electrical signal from photodetector 40 may beemployed directly. Alternatively, it has been observed by the presentinventors that the drive current passing through diode injection laser10 is, in fact, modulated by the light returning from the target.Accordingly, by providing a resistor R (reference numeral 90) in thecurrent path of power supply 70, it is possible to extract this samesignal as a voltage drop across resistor R by connecting its oppositeends to differential amplifier 50 which provides an alternate outputsignal. By integrating this signal over an appropriate time period τ(measured in milliseconds), it is possible to produce an output signal,also referred to herein as R (not to be confused with Resistor 90 havingresistance value R), which is proportional to the amount by which thelens should be repositioned in order to achieve optimal focus for aparticular layer.

As is more particularly illustrated in FIG. 3, at points of optimalfocus, the frequency of the dither modulated output signal is doubled.This phenomenon is more particularly described below with reference tothe discussions of FIG. 3.

FIG. 2 illustrates, in functional block diagram form, an apparatus whichwas employed to test the principles of the present invention. Inparticular, function generator 60 provided a dithering signal I=I₀ sinΩ_(D) t. At the same time, the laser optics setup which includedphotodetector 40, diode injection laser 10, beam collimating crossedfiber 80, and lens 100 resulted in a production of a signal S. Thedithering signal I and the return signal S are preferably passed throughamplifiers 55 and 56 respectively. The signal S can be represented as aFourier series with a DC component a₀ plus other sinusoidal andcosinusoidal components in accordance with the follow equation: ##EQU1##

The output from detector 40 or the voltage across resistor 90 isproportional to the vector cross product of the signals I and S. If thisproduct is integrated over the period of the dithering signal, namely,τ, the result that follows is shown below in equation 2. ##EQU2##

Because of the orthogonality relationships expressed in the equationbelow: ##EQU3## it is seen that the resultant output R is proportionalto the magnitude of coefficient a₁. This magnitude represents thedistance from the focal plane to the desired position. Accordingly, itseems to be a unique feature of the present invention that there isprovided a signal which is proportional to the amount by which thepositioning is in error. This is an important property because itprovides a feedback control mechanism for precisely aligning the lens.

It is further seen that the resulting output is either positive ornegative depending on the direction in which the lens is out of focus.The signal R, therefore, allows the determination of three conditions:(1) whether or not the system is out of focus; (2) the direction inwhich the lens must be moved to achieve focus; and (3) the amount bywhich the lens should be moved to restore focus.

The preceding equations further prove that it is always possible todetermine these properties for any amount of defocusing which occurs.This is a feature which is unique to the present invention especially incomparison with other systems which require that the lens be out offocus by a (possibly hefty) predetermined minimal amount. Such alimitation severely curtails fine control of lens focus. In the presentinvention, however, minimal correction limits are not an inherentlimitation.

More particularly, it is seen in FIG. 2 that the signal R may be fedback through optional inverter 110 to servo driver 200 which respondswithin the period τ to correctly position the lens for proper focus. Itis seen that this feedback mechanism employs the variable that is ofgreatest value in correcting the error. It is in this way that preciseand rapid alignment may be accomplished.

The principles upon which the present invention operates may be moreparticularly illustrated in FIG. 3 which shows a 5-layer optical diskexample. In particular, the lines extending axially outward from lens100 represents the cone of focus for that lens. In particular, for anygiven lens configuration, the code of focus possesses a point at whichit is narrowest. This point defines the waist plane for the lens. For aspecific lens shape, this plane is constant. It should also beparticularly noted with respect to this figure and others that therelevant feature is the relative distance between the lens and the disk.FIG. 3 has been constructed from the point of view of the lens. Thesolid vertical line in the center represents DC position of the lenswhen the dithering voltage applied is 0. This DC position can nominallybe thought of as the location of a layer within the disk to be read. Thenumbers shown in the left-hand portion of the figure correspond to thesignal time points shown in the right-hand portion of the figure. Theoperation of the dithering aspects are now particularly described.

In particular, it is seen in situation 1 in FIG. 3 that the DC positionis at its farthest right-hand travel. As the lens is dithered further tothe right, the signal level drops from the level at position 1 to itslowest point at position 2. As the dithered lens returns to the nominalDC position, the signal value returns to that which is shown at point 3in the diagram. As the lens position due to dithering moves furthertowards the right, that is, closer to the waist plane of the lens, thesignal increases to its highest positive output level at position 4.Again, thereafter, the signal level declines as the lens returns to itsDC position. Thus, as the lens is dithered over one cycle of duration τ,the signal levels vary to levels shown at positions 1, 2, 3, 4, and 5 inthe left-hand and right-hand portions of situation 1 ("1" as shown inthe circle in the figure) shown in FIG. 3.

As the DC position is moved further towards the left, it is seen that adifferent output signal is produced. This signal is shown in situation 2in FIG. 3. The first half of the cycle is almost identical to that whichis shown in situation 1. However, the depth of the curve is not assevere as it is in situation 1. Nonetheless, and more significantly,after the dithered position passes through the 0 bias position number 3,the dithered lens produces a signal which has twin peaks 4 and 6 as themaximal extent passes twice through the waist plane. The resultantoutput signal, therefore, illustrates a form of frequency doubling inthe latter half of the period τ. The resultant output signal is shown assituation 2 in FIG. 3.

It is to be particularly noted that in situation 3 in FIG. 3, the outputwaveform exhibits what is effectively a frequency doubling of thedithering driver signal. This occurs because the DC position isidentical to the waist plane. This illustrates the situation in whichthe focus on a plane is optimal.

This doubling of the dithering frequency is a phenomenon reported inU.S. Pat. No. 4,385,774, issued Nov. 9, 1982, to Richard L. Wilkinson.However, the teachings of Wilkinson do not provide a mechanism forextracting the value R which is indicative of the amount by which thelens is to be moved to achieve the desired focus. Without this variable,feedback control of the alignment mechanism is difficult to achieve.

With respect to the other situations shown in FIG. 3, situations 4 and 5are somewhat symmetric but mirror images of situations 2 and 1respectively. In each case the output signal waveform is shown.

The present inventors have further recognized that the resultant outputwaveforms may be processed in a digital fashion. In particular, thewaveforms shown in situations 1 through 5, in FIG. 3, may be passedthrough a threshold detector. If this process is carried out, theresultant signal waveforms and the form of individual spikes or pulsesare shown in FIG. 4, parts B through F respectively. In particular,referring to part B of FIG. 4, it is seen that situation 1 results intwo pulses below the 0 voltage reference line followed by two voltagepulses above the reference line as shown. These pulses result when theoutput signal form rises above reference levels 51 or 52 as shown in theleft-hand portion of FIG. 4. In a similar way, a sequence of pulses orspikes may be produced as a result of the other situations. These areshown in portions C, D, E, and F of FIG. 4.

Furthermore, the present inventors have realized that these signals maybe passed through a positive pulse counter and a negative pulse counterto provide a very rapid indication of where the DC position of the lensis with respect to the desired surface to be focused upon. A circuit forcarrying out this digital processing operation is shown in FIG. 5. Analternative is shown in FIG. 6. With specific reference to FIG. 5, thesignals from the apparatus shown in FIG. 1 are passed through thresholddetector 120 resulting in a series of spiked pulses as shown in theright-hand portion of FIG. 4. A positive pulse counter 130 is providedto count pulses that are positive in voltage. Likewise, negative pulsecounter 140 counts the negative pulses. The output from these countersare provided to state detector logic 150 which can provide an almostimmediate indication of the relative versus desired position of the DClens position.

In particular, if the positive pulse counter 130 has a value 1, thismeans that situations D, E, and F in FIG. 4 are possibilities. If thenext pulse is a positive pulse, situations E and F are possibilities. Ifthree positive pulses in a row occur, then it is clear that thesituation illustrated in part E of FIG. 4 pertains. This corresponds tosituation 4 in FIG. 3. It is, thus, then known that it would bedesirable to move the DC value of the dithering offset so as to move thelens to the right. It is to be particularly noted that not all of thepulses need to be generated before it is determined what has to be doneto change the DC offset value. This is very desirable in that it speedsthe correction operation. One does not even have to wait for a wholedithering period τ to begin producing the desired positioning via thefeedback servo loop. Alternatively, an up-down counter 135 may supplyits output signals to counter logic 155 which operates in a similarfashion to provide an indication of the lens position versus its desiredposition.

In the process of developing the improved focusing system disclosedherein, the present applicants have also discovered and utilized anentirely new phenomenon with respect to the operation of certain laserdevices. In particular, semiconductor diode lasers, such as thosefabricated either from solid state materials including aluminum galliumarsenide, may be made to function as light detectors when modulatedlaser light is reflected back into the laser cavity. Modulation of thelight reflected back into the cavity appears as a correspondingmodulation of the laser drive current. If the signal being detected isat a different frequency than the laser drive signal, such that the twocan be separated with a frequency selective filter, then the detectedsignal can be obtained from a resistor placed in series so as to conductthe drive current (as discussed above and shown in FIG. 1). Any otherconvenient means for detecting variation in current may also beemployed.

In particular, reference to the focusing application discussed herein,the dither signal is at a much lower frequency than the laser data rateand, as a result, it is thus very easily separable using conventionalsignal filtering devices and methods. However, the use of thisphenomenon is not restricted to CD-ROM-type applications. For example,it may be employed in barcode scanners or in fiber optic sensor systemswhich employ laser light reflected from a target.

The use of the above-described semiconductor laser devices as both laserlight source and as signal detector offers many advantages to theindustry. For example, this aspect of the invention clearly eliminatesthe need for an extra, separate photodetector. This alone is a decidedcost, packaging and manufacturing advantage. However, it is only one ofmany related advantages. Additionally, this design simplifies thefocusing optics and alignment since the light to be detected is simplyreflected back along the same path into the source from which it came.By virtue of this simplified optical geometry, the apparatus is moretolerant of shock, vibration and other sources of misalignment. Opticalor light amplitude techniques suffer from aberrations in the optics andfrom light source fluctuations. The present invention, however, employsa frequency method which is essentially free of these amplitudeproblems. It is, therefore, seen that a direct consequence of theutilization of frequency effects is that the overall system is much moreimmune to noise effects (such as, for example, light intensityfluctuations caused by variations in the laser drive current). This doesnot, however, preclude amplitude-based methods from being used inconjunction with the present invention.

Furthermore, since the photodetector has been eliminated and since theoptical tolerances have been improved, the reading head may be madelighter (less mass and weight). This means that the reading head may bemoved faster. This alone increases data access times and, therefore,data retrieval times. Moreover, by simplifying the focusing optics, thereading head may be placed closer to the CD-ROM disk. This makes itpossible to manufacture more compact drive units, a characteristic whichis especially important for portable applications.

Lastly, but very importantly, by being able to move the reading headcloser to the disk and achieving better focus of the light, one canplace the tracks closer together and can thus increase the storagedensity, even in single-sided, single-layer CD devices.

From the above, it should be realized that the present invention makesit eminently more practical to control the focusing of laser light ontovarious planes within a solid body. Thus, while the primary applicationof the positioning system of the present invention is directed tomultilayer CD-ROM reading devices, it should also be appreciated thatthe devices and methods disclosed are applicable to readingthree-dimensional holographic information. Thus, the present inventionis also a control mechanism for accurately selecting a planar region ina solid body used as a holographic storage medium for interactionbetween the laser light and the chemical, crystalline, structural orother properties of the solid body which are present in the selectedplane.

Yet another utilization of the principles of the present invention canbe found in a system for improved CD-ROM track following in the X-Yplane, as opposed to Z-direction position control as described above. Inthis case, dithering is preferably applied in the radial directionrelative to the curved tracks on a CD-ROM disk. Since the between-trackarea is more reflective than the pitted written section, advantage maybe taken of the nature of the returned signal to indicate the positionof the focused laser spot relative to the track to be followed. Asabove, this greater degree of control allows a closer spacing of thetracks and thus provides an increase in areal data density. However,because there are portions of the tracks that may not be written, orwritten with various pitted data patterns, it is preferable to averagethe output level over a longer period of time. Thus, for example, whilethe integration process described above is performed over a period τ,the process for X-Y direction control may employ integration over aperiod of 4τ, for example.

The principles of X-Y positioning are illustrated in FIGS. 7A through7D. In each case, there is shown the median position of a spot offocused (laser) light which is being dithered left and right(X-direction) at a frequency ω. If, as shown in FIG. 7A, the spot doesnot ever cross the boundary between a reflective area (on the left) anda non-reflective area (on the right), the output level is relativelyconstant (except for the pits in the non-reflective area which arecompensated for by the above-described averaging). If, as shown in FIGS.7C and 7D, the median position of the focused spot is as shown, theresulting output waveform changes as seen in the right-hand portions ofFIGS. 7C and 7D, respectively. Thus, as with Z-direction positioning,dithering can also be used to discriminate between desirable (FIG. 7B)tracking and the need to move the spot. Also, as above with respect toZ-direction positioning, the present invention also provides aparticular value, R, indicative of the amount by which the median spotposition is to be moved and the direction in which it must be moved,that is, to the right in FIG. 7C and to the left in FIG. 7D.

Yet another application of the present invention is found in the readingof stored (typically digital) information on an optical disk when thedisk contains both audio and video and/or audio and digital data. Inparticular, CD-ROMs for computer use and optical disks described asDigital Video Disks (DVDs) store information in different formats.Moreover, an optical disk could store audio information in one layer anddigital data on another layer. The laser spot size used for readingthese two different formats (audio and data) is also different. However,the present invention is capable of controlling both the plane of focusand the laser spot size. Even if these different formats are present ona single-layer optical disk, the disk is still fully readable by themethods and systems of the present invention.

It should also be appreciated that, while the example discussions hereinhave been directed to the utilization of a single read head, the presentinvention is nonetheless fully employable in situations and deviceswhich use an array of read heads (or other readout devices) to achievethe readout of data in parallel. In fact, the smaller sizes, madepractical by the present invention, provide greater room for additionalread heads for parallel operations.

From the above, it should be appreciated that all of the above-mentionedobjectives have been achieved. Furthermore, it is seen that theapparatus of the present invention provides a mechanism for preciselyaligning the focal point of a laser light source within a multilayeroptical storage disk device. Because the present invention can takeadvantage of waist-coil-activated mechanisms already in place forpositioning optical storage disk lens systems, the system of the presentinvention may be retrofitted to existing CD-ROM devices with minimalinconvenience. However, it is to be particularly noted that theoperation of the present invention is not limited to oscillatorymovement of the lens. The light source or the target may also be moved.What matters is the relative motion of one component in the system(light source, lens, reflective target) with respect to the other twocomponents. Motion of the target itself, though, is not desirable in thetypical CD-ROM system employing the present invention.

While the invention has been described in detail herein in accordancewith certain preferred embodiments thereof, many modifications andchanges therein may be effected by those skilled in the art.Accordingly, it is intended by the appended claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

The invention claimed is:
 1. A combined laser source and detector systemcomprising:a semiconductor laser light source which is capable of havingits light output modulated by varying drive current applied to saidsemiconductor light source; means for varying the focus of light fromsaid source, in an oscillatory manner, at a selected frequency; andmeans for detecting, in the range of said selected frequency, variationsin current flowing through said semiconductor source, said variationsbeing induced by laser light which is emitted from and is subsequentlyreturned to said semiconductor light source.
 2. The system of claim 1wherein said means for detection comprises a resistive means throughwhich said current passes and a frequency filter means which is coupledto voltage signals appearing across said resistive means and which iscapable of selectively passing signals in the same frequency range assaid variations.
 3. The system of claim 1 further including reflectivetarget means which is positioned to return light from said source backto said source.
 4. The system of claim 3 in which said reflective targetis rotating.
 5. The system of claim 3 in which said target is a compactdisk.